1. Field of the Invention
The invention relates to a process and an apparatus for continuous extrusion of rods of plasticized raw material provided with at least one internal channel which is helical in at least portions.
2. Discussion of the Background
Such a process as well as apparatuses, or in other words extrusion heads, for performing such a process are used, for example, when a rod preform of a plasticized powder compound, such as a powder-metallurgical compound, or in other words a hard-metal or ceramide compound, is to be shaped to a preform, or in other words a sintered-metal or a sintered-ceramic preform, from which a preform in the form of a cylindrical rod for a high-performance tool is then produced in a sintering or baking process. By virtue of the dies or powder mixtures used, these preforms are characterized by extremely high basic strength, especially as regards mechanical stresses and strains as well as abrasion, and so a trend has developed toward using such preforms especially in the manufacture of drilling or milling tools. Since these tools are frequently operated with extremely high cutting speeds, it is important that the lubricant being used be supplied selectively and in many cases under very high pressures to those regions of the cutting edges which are subject to the highest stresses and strains. This is best ensured by co-formed, internal cooling channels, which then emerge on a predetermined pitch circle on the front end of the tool, or in other words preferably on a flank of the ground surface of the tool. Because sintered hard-metal tools can be machined only with costly methods, it is desirable that the shape of the preform approach the final geometry of the tool as closely as possible. This is achieved most simply by using an extrusion process, with which the capability exists of making the preform such that it already has finish-formed internal cooling channels in a continuous process, which has the particular additional advantage that tools of very great length can be made without changing the process operation.
Certainly the process is economical only if it is possible to make the rod such that the geometry and in particular also the position of the at least one internal lubricant or cooling channel are kept within very narrow tolerance limits. This problem becomes more acute if the tool to be made--as is the case for a drilling tool, for example--must be provided with clamping grooves. Because fully hard metal drilling tools are now made with relatively large axial lengths, the at least one internal cooling channel must be co-formed sufficiently exactly that it is disposed at exactly the predetermined position in the drill web in each cross section of the drilling tool, since only then is it ensured that the drill stability will be constant over the entire length and that the exit point of the internal cooling channel will remain unchanged relative to the main cutting edge during finish grinding of the tool.
There are already known numerous attempts to make such tool preforms from plasticized powder compound in the extrusion process in such a way that the requirements of accuracy of position and form of the internal cooling channels are met.
In U.S. Pat. No. 2,422,994 there is already described an extrusion process in which a plasticized powder-metallurgy compound is extruded through an extrusion nozzle. The inside surface of the extrusion nozzle has projections of predetermined cross section and, in the region of the center of the nozzle, there extend axially oriented, rod-like elements fixed to a core, which is disposed upstream from the extrusion nozzle and around which the plasticized compound flows. This process operates in multiple stages, wherein the plasticized raw material is first shaped to a preform with at least one straight, external groove, whereupon the preform obtained in this way is twisted by a relative rotational movement between the extrusion nozzle and the raw material. Such a two-stage forming process is already unfavorable for most raw compounds that have come into use since then, however, because the preform emerging from the extrusion nozzle is extremely pressure-sensitive. Even very small external forces acting on the preform would cause undesired large deformations, especially of the internal co-formed channels, whereby the preform would immediately become unusable.
To overcome this problem, German Patent DE 3601385 describes an extrusion process in which the helical profile of the at least one internal coolant channel is produced simultaneously with the extrusion of the plasticized compound, although in this case the nozzle mouthpiece must be provided internally with a helical profile. At the center of the extrusion nozzle there are provided elastic pins, which at their upstream ends are fixed to a nozzle core and the elasticity of which is chosen to be sufficient that the pins can conform to the swirl flow induced by the internal contour of the nozzle mouthpiece. It has been found that it is difficult to form the cooling-channel helix sufficiently accurately in the preforms with this extrusion head. The projections and depression on the inside surface of the nozzle mouthpiece had to be provided in large numbers in order to induce appropriate rotation of the flow of the compound. As a result, the nozzle mouthpiece is relatively expensive and, moreover, the projections present on the sintered preform must be ground off first of all, leading to material losses.
In European Patent Application EP 465946 A1 there are described a process and an apparatus according to the preamble of claim 1 or of claim 5, with which it is possible to eliminate the process step of cylindrical surface grinding of the finish-sintered cutting-part preforms. For this purpose, the inside surface of the nozzle mouthpiece is formed by the envelope surface of a regular cylinder. A swirl device disposed in the flow of the compound is mounted upstream from the nozzle mouthpiece. Corresponding to one alternative, a swirl motion acting uniformly over the cross section of the strand is imparted to the extrusion compound by means of this swirl device whereas, according to a second alternative, a spinning or revolving movement is imparted to the swirl device by the extrusion compound. To form the at least one internal channel, a filament-like material which conforms to the swirl or rotational motion penetrates into the flow of compound. Thus the pitch-circle diameter on which the cross section of the at least one internal cooling channel is eventually disposed in the extruded preform is influenced by the flow velocity and by the friction losses in the nozzle mouthpiece. According to a further variant of this known process, it is therefore proposed that the nozzle mouthpiece be designed to revolve, thus allowing the rotational motion of the flow of compound to be corrected by the revolving movement.
With this known process, plasticized compounds can be processed in the extrusion process to preforms which are characterized by extremely high accuracy as regards their outside dimensions and the geometry and position of the at least one internal cooling channel. Of course, in this known process and these known apparatuses for performing the process, there exists the need to keep the working accuracy largely independent of the operating parameters of the process, such as the flow conditions in the inlet region of the nozzle mouthpiece, the composition of the plasticized compound and the flow velocities through the nozzle mouthpiece, etc.